Galvanic bath

ABSTRACT

The invention relates to a galvanic bath for depositing a nickel-molybdenum alloy which consists of an aqueous solution of amine complexes and/or ammonium complexes in each case of nickel and/or molybdenum, the galvanic bath containing citric acid and/or citrate ions and/or oxidation products of the citric acid and/or the citrates, and molybdenum being present in different oxidation stages, in particular as Mo(V) and Mo(VI), in the galvanic bath.

The invention relates to an electroplating bath for depositing nickel-molybdenum alloys on a surface of an object.

Contact elements of plug connectors, for example, can be coated with such nickel-molybdenum alloys. Further layers, for example consisting of silver/silver alloys or gold/gold alloys or copper/copper alloys, can be applied to a nickel-molybdenum alloy.

PRIOR ART

Electroplating baths based on ammonium-nickel and ammonium-molybdenum complexes are generally considered to have low stability in the prior art. In the baths studied, insoluble molybdenum compounds, for example MoO(OH)₃ (trihydroxomolybdenum monoxide) are formed, as a result of which the electrolyte becomes unusable.

DE 121664 B, JP 2005-082856 A, JP 04124293 A disclose electroplating baths for depositing a nickel-molybdenum alloy. The electroplating baths additionally contain ammonium and/or citrate ions.

A disadvantage of these baths is considered to be that the current yield and the composition of the layer change continually during the coating process and the process becomes virtually uncontrollable as a result. Molybdenum oxides are included in the layer and coatings having only low shine are deposited. For these reasons, the baths having this composition have hitherto been without industrial importance.

FORMULATION OF PROBLEM

The problem addressed by the present invention is to provide a stable alloy bath which makes it possible to deposit nickel-molybdenum alloys having a high molybdenum content.

The problem is solved by an electroplating bath as claimed in claim 1.

Advantageous embodiments of the invention are indicated in the dependent claims.

The electroplating bath of the invention can be produced in a simple way. In addition, it is environmentally friendly.

An electroplating bath can consist of an aqueous, preferably alkaline (basic), solution of amine complexes and/or ammonium complexes, in each case of nickel and/or molybdenum, for example [Ni(NH₃)₆]²⁺ or (NH₄)₆Mo₇O₂₄, and many salts of the corresponding alloy-forming metals. These baths display long-term stability and can be used for the electrolytic deposition of nickel-molybdenum alloys.

The addition of a citrate prevents the electroplating bath from becoming unusable as a result of the above-described precipitation of insoluble molybdenum compounds. The citrate suppresses the disportionation reaction of the molybdenum complexes.

Instead of citric acid and/or citrates, it is also possible to use their oxidation products which are formed by thermal and/or anodic oxidation of the citric acid/citrates. As oxidation products, the bath contains, for example, ketoglutaric acid and/or aconitic acid and/or α-ketoglutarates and/or β-ketoglutarates and/or aconitates. The abovementioned materials preferably have a concentration in the range from 0.1 to 0.6 mol/l, particularly preferably from 0.1 to 0.4 mol/l, in the bath.

It is particularly advantageous for molybdenum to be present in different oxidation states in the electroplating bath.

The soluble complexes of molybdenum can be, before being used according to the invention, prepared by, for example, reacting molybdenum salts in which the molybdenum is present in different oxidation states in aqueous solution at room temperature with a complexing agent in a molar ratio of 1 mol of molybdenum to from four to 10 mol of complexing agent. However, it is also possible to add the molybdenum salts and complexing agents directly to the bath solution.

The molybdenum in different oxidation states is generated by chemical and/or electrochemical reduction of molybdate ions. After reduction, molybdenum ions in the oxidation states +3, +4, +5 and +6, preferably +5 and +6, are present in the solution.

Salts of nickel and/or cobalt and/or iron and/or phosphorus and/or rhenium and/or palladium and/or platinum, which in combination with molybdenum salts allow various alloy compositions of the layer, are advantageously used as electrolyte. Up to 50% by weight of molybdenum can be deposited in this way.

The molybdenum content of the alloy matrix exercises a substantial influence on the structure of a metal coating. Thus, for example, scanning electromicrographs (SEM) have shown that a layer having a molybdenum content of from 20 to 40 percent by weight (% by weight) has finely crystalline to amorphous structures.

The bath can advantageously contain organic additives such as stabilizers, wetting agents and brighteners. The customary wetting agents are nonionic, cationic or anionic in nature. These materials can also act as brighteners in concentrations of from 0.01 to 20 g/liter.

The electroplating bath advantageously contains at least one additive from the following group or a mixture thereof:

-   -   brighteners, preferably in a concentration in the range from         0.01 to 5 percent by weight (% by weight),     -   wetting agents, preferably in a concentration in the range from         0.05 to 0.5% by weight. The customary wetting agents are         nonionic, cationic or anionic in nature.     -   electrolyte salts, preferably in a concentration in the range         from 0.2 to 0.8 mol/l, particularly preferably in a         concentration in the range from 0.3 to 0.6 mol/l.

As wetting agents, it is advantageous to use lauryl sulfate, lauryl ether sulfate or acrylamido sulfonates or a mixture of the above-mentioned wetting agents. These wetting agents greatly reduce the surface tension of the bath. Visually defect-free and high-quality coatings can be achieved by means of these additives.

Sulfonimides, sulfonamides, alkylsulfonic acids (sulfonates), arylsulfonic acids (sulfonates) or a mixture thereof are ideal as brighteners for the nickel-containing baths.

As electrolyte salts, it is advantageous to use sodium and potassium salts. The preferred concentrations of these materials are in the range from 0.2 to 0.8 mol per liter (mol/l), but preferably in the range from 0.3 to 0.6 mol/l.

The pH of the electroplating bath is advantageously in the range from 4 to 11, but particularly preferably in the range from 7.5 to 9.5. The pH is advantageously set by addition of alkali metal hydroxide, for example NaOH. It has been found that the abovementioned citrate is particularly well suited to suppressing disproportionation in these pH ranges.

The electroplating bath is advantageously operated in a temperature range from 20 to 85 degrees Celsius (° C.), in particular from 50° C. to 75° C.

Current densities in the range from 0.1 to 3 ampere per square decimeter (A/dm²) are preferably employed in the coating operation.

Ketoglutaric acid and/or aconitic acid and/or α-ketoglutarates and/or β-ketoglutarates and/or aconitates are particularly preferably present in the electroplating bath, here the concentration of these materials is preferably in the range from 0.1 to 0.6 mol/l, particularly preferably from 0.1 to 0.4 mol/l.

The electroplating bath of the invention is outstandingly suitable for depositing silver-colored nickel-molybdenum alloys on industrial objects, for example abrasion-resistant and corrosion-resistant coatings on electronic components. Contact elements of plug connectors can particularly advantageously be coated therewith. The electrolytically deposited coating is particularly corrosion- and wear-resistant.

The electroplating bath of the invention displays long-term stability and can be used for the electrolytic deposition of nickel-molybdenum alloys. Such a result was not able to be achieved by the known baths having a similar composition.

In the following, the essence of the electroplating bath of the invention will be summarized once more. It is provided as an aqueous solution of salts or oxides of nickel and/or of molybdenum and further additives. Furthermore, many alloy-forming metals can be added in ionic form. The pH of the bath is set to the weakly to strongly alkaline region. Since nickel and molybdenum should be present in the form of amine or ammonium complexes, amine- or ammonium-containing compounds are added to the bath. An example is [Ni(NH₃)_(n)]²⁺ where n=1-6.

Further suitable nickel sources are nickel (II) sulfate, nickel sulfate hexahydrate or nickel chloride. The nickel concentration in the bath is advantageously in the range from 0.20 to 0.35 mol/l, particularly preferably from 0.22 to 0.3 mol/l. 

1. An electroplating bath for depositing a nickel-molybdenum alloy, said bath comprising an aqueous solution of amine complexes and/or ammonium complexes, in each case of nickel and/or molybdenum, wherein the electroplating bath contains a citric acid and/or citrate ions and/or oxidation products of citric acid and/or of citrates, and wherein molybdenum is present in different oxidation states, in the electroplating bath.
 2. The electroplating bath as claimed in claim 1, wherein the concentration of the citric acid and/or of the citrate ions and/or the oxidation products of citric acid and/or of citrates is in the range from 0.1 to 0.6 mol/l, in particular from 0.1 to 0.4 mol/l.
 3. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains a proportion of molybdenum of at least 15% by weight.
 4. The electroplating bath as claimed in claim 1, wherein the concentration of nickel is in the range from 0.20 to 0.35 mol/l, in particular from 0.22 to 0.3 mol/l.
 5. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains an ammonium concentration in the range from 0.20 mol/l to 0.40 mol/l, preferably from 0.25 to 0.35 mol/l.
 6. The electroplating bath as claimed in claim 1, wherein the concentration of molybdenum is in the range from 0.01 to 1 mol/l, in particular from 0.02 to 0.06 mol/l.
 7. The electroplating bath as claimed in claim 1, wherein the molybdenum/nickel ratio is in the range from 1:10 to 1:4.
 8. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains further salts of alloy-forming metals, for example salts of nickel and/or cobalt and/or iron and/or phosphorus and/or rhenium and/or palladium and/or platinum, and where the individual metals are preferably present in a concentration in the range from 0.1 to 5% by weight.
 9. The electroplating bath as claimed in claim 1, wherein the bath contains organic additives such as stabilizers, wetting agents and brighteners.
 10. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains at least one additive selected from the following group or a mixture thereof: brighteners, preferably in a concentration in the range from 0.01 to 5% by weight, wetting agents, preferably in a concentration in the range from 0.05 to 0.5% by weight, electrolyte salts, preferably in a concentration in the range from 0.2 to 0.8 mol/l, particularly preferably in a concentration in the range from 0.3 to 0.6 mol/l, sulfur-containing additives, preferably in the concentration range from 0.1 mg/l to 4.0 g/l, particularly preferably in the concentration range from 0.2 mg/1 to 2.0 g/l.
 11. The electroplating bath as claimed in claim 1, wherein electrolyte salts such as sodium salts and potassium salts are present in the electroplating bath.
 12. The electroplating bath as claimed in claim 1, wherein the concentration of the electrolyte salts is in the range from 0.2 to 0.8 mol/l, but preferably in the range from 0.3 to 0.6 mol/l.
 13. The electroplating bath as claimed in claim 1, wherein the electrolyte salt is an inorganic electrolyte salt, in particular from the group consisting of sulfates and chlorides, or an organic electrolyte salt, in particular from the group consisting of citrates.
 14. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains ketoglutaric acid and/or aconitic acid and/or ketoglutarates and/or aconitates, and where the concentration of these materials is preferably in the range from 0.1 to 0.6 mol/l, particularly preferably from 0.1 to 0.4 mol/l.
 15. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains gluconates, tartrates or hydroxycarboxylic acids.
 16. The electroplating bath as claimed in claim 1, wherein sulfur-containing additives such as sulfanilamides, sulfonimides, sulfonic acids or sulfonates have been added to the electroplating bath, and where the concentration of these sulfur-containing additives is preferably in the range from 0.1 mg/l to 4.0 g/l, in particular from 0.2 mg/l to 2.0 g/l.
 17. The electroplating bath as claimed in claim 1, wherein at least one surfactant from the group consisting of cationic, anionic, nonionic, amphoteric surfactants, or mixtures of the abovementioned surfactants, is present as wetting agent in the electroplating bath.
 18. The electroplating bath as claimed in claim 1, wherein the pH of the electroplating bath is in the range from 4 to 11, but particularly preferably in the range from 7.5 to 9.5.
 19. The electroplating bath as claimed in claim 1, wherein the electroplating bath contains alkali metal hydroxide for adjusting pH.
 20. The electroplating bath as claimed in claim 1, wherein the electroplating bath has a temperature in the range of from 20 to 85° C., in particular from 50° C. to 75° C.
 21. An object, preferably a contact element of a plug connector, the surface of which has been coated with a nickel-molybdenum alloy using an electroplating bath as claimed in claim
 1. 22. The electroplating bath as claimed in claim 1, wherein the molybdenum is present as Mo(V) and Mo(VI). 